Long recognized as crucial elements of controlling a boiler are fuel/air mixing, fuel/air ratio and achieving ignition energy for combustion. Optimal combustion is stable, has high energy conversion of the fossil fuel to CO2, minimum NOx and CO in the flue gas, and minimum harmful intermediaries (such as Hydrogen Sulfides) on the combustion walls. The financial benefits of low excess air operation are significant and include increased efficiency and cost savings in air pollution control equipment. Historically, boilers were designed to combust at 18 to 20% excess air to insure full combustion of the fossil fuel occurred. However, it has been demonstrated that operating at lower amount of excess air, for example 12%, can achieve a significant cost savings. The fundamental limitation in operating with low excess air is incomplete combustion resulting in increased CO along with hazardous air pollutant (HAP) emissions. Consequently, it is critical to have a gas monitoring system capable of continuously and accurately measuring the composition of the flue gas exiting the boiler, such as the oxygen.
Unfortunately, current oxygen monitoring systems are insufficient to operate a boiler at lower excess oxygen levels in the flue gas because the oxygen sensors and methods for measuring the oxygen within the flue gas do not provide a complete and reliable measurement of the oxygen level and have a tendency to drift which may cause the boiler to deviate from optimal operation.
Currently, existing commercial oxygen probes measure the oxygen concentration at only one, or only a few locations. Optimizing boiler combustion requires a good understanding of the oxygen concentration and gas temperature across a large gas duct (up to 30 feet wide). To achieve a good understanding of the oxygen concentration and temperature within a duct, including imbalances across the duct, experience has shown that a minimum of 12 to 16 measurement location is required. Installing 12 or more individual zirconium oxide sensors to measure O2 concentrations, plus an equal number thermocouples to measure temperature involves many separate pieces of equipment/sensors which must be mounted, connected, powered, and maintained. The total installation cost of such a system is large, and prevents many boilers from being fully instrumented with traditional sensors.
Similarly, extractive gas measurement systems are widely used for boiler performance and emissions testing. However, these systems are generally not suited for long-term continuous operation for several reasons. Many of the chemical analyzers typically used may not support long term continuous operation, or require very frequent maintenance. Filters on the gas extraction grid clog, and require periodic cleaning. Gas cooling and drying equipment is required before the extracted boiler gas can be fed to many chemical analyzers, and this equipment requires frequent maintenance. Many of the extractive systems sample from a single location at a time, and cannot provide multiple simultaneous measurements at each sample location.